User blog:Samuli.seppanen/Plans for improved cones
In my previous blog post I mentioned that cones start bending visibly when power levels are ramped up. Most of the bending in the cones is concentrated to the area halfway between the hook and the outer half of the torsion spring. What this means is that there's a stress concentration point in there caused by suboptimal tillering (=the way cones are tapered). For a thorough explanation on how tillering works please refer to the Bow design article in the Crossbow building wiki. In the cheiroballistra case the cones taper in all directions, not just in width, meaning that a linear taper will automatically create a stress concentration at the middle and reduce the durability of the arms*. What I need to do is make new cones where the tapering angle increases when moving towards the tip, basically move to using a more elliptical tiller. This will strengthen the currently over-bending middle part and greatly increase the durability of the cones. There is also a small but perceptible amount of permanent bend in the cones, which is due to the bellies of the cones crushing under compressive stress. The underlying reason is the round cross-section of the cones: during pullback very small surface areas at the back and the belly have to endure overly high levels of the stretch and compression, respectively. A more rectangular cross-section would be much stronger as the same amount of stress would be divided to a much larger surface area. A completely rectangular cross-section won't be possible because the hoop would crush the corners of the cone and the cone's edges would chafe the torsion spring cords, but a heavily rounded square cross-section is certainly doable. When these new arms are ready the cheiroballistra should be ready for the 50% power increase. At that point I'll see what part - if any - breaks next. If all goes well the cheiroballistra has reached fairly acceptable performance levels as far as I'm concerned. Here are my rough estimates: * 30 gram bolts: 83.01 m/s, 103 joules (energy increase factor 1.45) * 20 gram bolts: 92.20 m/s, 92 joules (energy increase factor 1.4) * 10 gram bolts: 104.4 m/s, 54.5 joules (energy increase factor 1.35) As the velocity of the bolt increases the relative efficiency drops, because a fast-moving bolt requires fast-moving arms and a fast-moving bowstring, both of which are basically just useless, dead mass. This also means that as the bolt size is decreased, the amount of dead mass compared to the amount of bolt mass increases, and the relative efficiency of energy transfer is reduced. Even with the current data it's safe to say that the cheiroballistra is designed to shoot bolts that weight 20-30 grams, depending on the range required. The tiny 10 gram bolts fly really fast even now, but the amount of punch they pack is fairly limited especially at the ranges where the heavier bolts could not be used. The maximum range with 20 gram bolts would probably be around 400-500 meters. In any case, once the new cones have been finished the cheiroballistra will clearly surpass the performance of the Wilkins' (2000: 93) huge, badly designed winched construction in all areas and will weigh less than a third of it. Basically the velocity of reasonably sized bolts will be double and the energy levels will be the same; for more details please refer to the Personal torsion weapons wiki page. * This was certainly known by the Romans who produced ballista arms. I would even go as far to suggest that the people who made the cones were either bowyers, or otherwise had similar set of skills. The thing is that if you don't understand these things your cones will inevitably explode at high power levels, even if they work ok at low stress levels. Category:Blog posts Category:Cheiroballistra Category:Theoretical Category:Woodworking Category:Arms Category:Practical Category:Backup